The present invention relates to a developer for a copier, facsimile transceiver, printer or similar image forming equipment and using a one component type developer, i.e., a toner.
With image forming equipment of the type forming an electrostatic latent image on an image carrier and then developing it by a developer, it is advantageous to use a developing device operable with a one component type developer, i.e., a toner in respect of the size, cost, reliability, etc. Regarding color images, use is advantageously made of a nonmagnetic toner having inherently high clearness. To deposit a predetermined charge on the toner and transport it to a developing region where the image carrier is located, the developing device may be provided with a developer carrier and a developer supply member for feeding the toner to the developer carrier. The developer carrier is driven such that the surface thereof passes a position where the developer carrier faces the image carrier. For example, Japanese Patent Laid-Open Publication No. 42672/1986 discloses a developing device having a developer carrier in the form of a developing roller of medium resistance (10.sup.9 .OMEGA.cm to 10.sup.11 .OMEGA.cm) and having a float electrode, and a developer supply member implemented as a sponge roller made of, for example, polyurethane. The developing roller and sponge roller are pressed against each other and rotated such that their surfaces move in opposite directions at the position where they contact. A blade, or layer forming member, is pressed against the developing roller at a predetermined pressure to cause the toner to deposit on the roller in a predetermined amount. The toner conveyed to the contact position by the sponge roller is frictionally charged at the contact position and then deposited on the developing roller. The blade regulates the toner on the developing roller to form a toner layer having a predetermined thickness. The developing roller transports the regulated toner layer to a position where the roller contacts a photoconductive element, or image carrier, thereby developing a latent image electrostatically formed on the element.
Regarding the developing system using a one component type developer, e.g., a nonmagnetic one component type developer, an optimal amount of charge and an optimal amount of deposition of the toner are as follows. Preferably, the amount of charge should be 5 .mu.c/g to 10 .mu.c/g in mean value, and the charge distribution should be stable, i.e., contain a minimum amount of relatively low charge toner which would reduce sharpness and resolution and contaminate the background. On the other hand, the toner deposition on the developing roller should preferably be such that the toner deposits on the image carrier in an amount of about 0.6 mg/cm.sup.2 to about 1.0 mg/cm.sup.2 or deposits on a recording medium in an amount of about 0.5 mg/cm.sup.2 to about 0.7 mg/cm.sup.2. The amount of toner deposition on the image carrier and recording medium are affected not only by the amount of toner on the developing roller but also by the relative speed of the image carrier and developing roller in the developing region.
However, the problem with the conventional developing device is that the toner is deposited on the developing roller only in a single layer. Specifically, while the charge deposited on the toner to reach the developing region is about 5 .mu.c/g to about 15 .mu.c/g in mean value, the amount of toner deposition on the developing roller is as small as 0.2 mg/cm.sup.2 to 0.8 mg/cm.sup.2. It follows that a desired amount of toner cannot be deposited on, for example, the image carrier unless the developing roller is rotated at twice to four times higher speed than the image carrier. When the rotation speed of the developing roller is increased to compensate for the short amount of toner on the developing roller, it is difficult to increase the image forming speed. Moreover, the higher rotation speed undesirably increases the density at the trailing edge portion of a solid image. Although this kind of phenomenon does not matter at all when a black-and-white image is produced, it increases the density at the trailing edge portion of a color image since a color is perceived through the toner. Particularly, when a plurality of color images are superposed to form a composite color image, the colors are brought out of register.
Therefore, to achieve a desired amount of toner deposition on, for example, the image carrier without the above-stated local increase in density, it is necessary to drive the developing roller at a speed close to the speed of the image carrier, i.e., to effect substantially equispeed development and to deposit a greater amount of toner on the developing roller than conventional. Specifically, to deposit a sufficient amount of toner on the image carrier and recording medium by equispeed development, it is necessary that the toner be deposited on the developing roller in an amount of at least 0.8 mg/cm.sup.2 in the case of contact type development or in an amount of at least 10 mg/cm.sup.2 in the case of noncontact type development. The contact type development is higher in developing efficiency than the noncontact type development. Such an amount of toner deposition on the developing roller is not achievable unless the toner forms two or more layers on the roller.
Two or more toner layers will be achieved only if the previously mentioned blade is pressed against the developing roller at a lower pressure. However, this approach is not desirable for the following reasons. In the conventional developing device, the developing roller and the sponge roller are moved in opposite directions at the position where they contact, as stated earlier. Hence, an uncharged toner is also fed to part of the surface of the developing roller having moved away from the contact position by the sponge roller. As a result, an upper toner layer formed on the developing roller and reaching the position where the blade contacts the roller contains a great amount of uncharged toner. It follows that the charge distribution of the toner existing in part of the developing roller having moved away from the blade is less than 10 .mu.c/g and, moreover, uncharged toner and toner charged to opposite polarity are contained. The uncharged toner cannot be transferred in a desirable manner and, therefore, contaminates the background and lowers the resolution.
As stated above, the key to a high image forming speed and the equispeed development which eliminates the local increase in image density is to form on the developing roller two or more toner layers with a stable charge distribution, i.e., with no uncharged toner even in the uppermost layer and having a mean amount of charge ranging from 5 .mu.c/g to 10 .mu.c/g.